Double Onslaught

I'm a Raider with Rapid Assault and Avatar of the Chase, granting permanent Onslaught, which is not represented visually or with an aura icon, but I can see reflected in my movement speed and sheet damage.

I also have the passive node Graceful Assault, which, among other things, grants a 10% chance to gain Onslaught for 4 seconds on kill.

Even though I have permanent Onslaught from ascendancies, I'm still receiving the periodic 4-second Onslaught buff from Graceful Assualt, as represented by an icon at the top of my screen, the change in visuals on my character, and an increase to sheet dps and movement.

I was under the impression that Onslaught was a mechanic that was not supposed to stack. Am I mistaken or is this a bug?